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# reorder an array using only swap(ARRAY, X, Y),
# given a new set of indices
...
  my ($a, $x, $y) = @_;
  @$a[$x,$y] = @$a[$y,$x];
}

  for (0 .. $#$o) {
    my $idx = $$o[$_];
...
    (@P[$R[$_],$idx], @R[$P[$idx],$_]) =
    (@P[$idx,$R[$_]], @R[$_,$P[$idx]]);
  }